When you look at the age of the semi finalists, Djoko and Murray are 23 y-o, Rafa 24, and Fed 29, we can safely say that “grandpa” does more than resist and that when he plays well, he is very hard to beat.

You can just as well say, ignoring age, that when Murray plays well he is very hard to beat. Don’t forget, just as Nadal’s wins over Federer are mostly on clay, Murray’s wins over him are in three setters, which happens to be the format of this final!
Of course Fed can’t be written off, but this week, neither can Murray.